General Questions

Yes, the shattered comet Shoemaker-Levy 9 will collide with Jupiter over
a 5.6 day period in July 1994. The first of 21 comet fragments is expected
to hit Jupiter on July 16, 1994 and the last on July 22, 1994. The 21 major
fragments are denoted A through W in order of impact, with letters I and O
not used. All of the comet fragments will hit on the dark farside of Jupiter.
The probability that all of the comet fragments will hit Jupiter is greater
that 99.9%. The probability that any fragment will impact on the near side
as viewed from the Earth is less than 0.01%.

The impact of the center of the comet train is predicted to occur at a
Jupiter latitude of about -44 degrees at a point about 67 degrees east
(toward the sunrise terminator) from the midnight meridian. These impact
point estimates from Chodas and Yeomans are only 5 to 9 degrees behind the
limb of Jupiter as seen from Earth. About 8 to 18 minutes after each
fragment hits, the impact points will rotate past the limb. After these
points cross the limb it will take another 18 minutes before they cross the
morning terminator into sunlight.

Comet Shoemaker-Levy 9 (1993e) was the ninth short-period comet discovered
by Eugene and Carolyn Shoemaker and David H. Levy and was first identified on
photographs taken on the night of 24 March 1993. The photographs were taken
at Palomar Mountain in Southern California with a 0.46 meter Schmidt camera
and were examined using a stereomicroscope to reveal the comet [2,14]. The
13.8 magnitude comet appeared 'squashed' in the original image. Subsequent
photographs taken by Jim Scotti with the Spacewatch telescope on Kitt Peak
in Arizona showed that the comet was actually a shattered comet. Some
astronomers call the comet a "string of pearls" since the comet fragments
are strung out in a line or train.

Before the end of March 1993 it was realized that the comet had made a very
close approach to Jupiter in the summer of 1992. At the beginning of April
1993, after sufficient observations had been made to determine the orbit more
reliably, Brian Marsden found that the comet is in orbit around Jupiter. By
late May 1993 it appeared that the comet was likely to impact Jupiter in 1994.
Since then, the comet has been the subject of intensive study. Searches of
archival photographs have identified pre-discovery images of the comet from
earlier in March 1993 but searches for even earlier images have been
unsuccessful. See [11] for more information about the discovery.

Jupiter will be about 770 million kilometers (480,000,000 miles) from
Earth, so it will be difficult to see the effects from Earth. Also,
the comet fragments will not effect Jupiter as a whole very much. It will
be like sticking 21 needles into an apple: "Locally, each needle does
significant damage but the whole apple isn't really modified very much." [35].
The energy deposited by the comet fragments fall well short of the energy
required to set off sustained thermonuclear fusion. Jupiter would have to
be more than 10 times more massive to sustain a fusion reaction.

Calculations by Paul Chodas (JPL) indicate that as seen from the Earth,
the fragments will disappear behind the limb of Jupiter only 5 to 15 seconds
before impact. The later fragments will be visible closer to impact.
Fragment W will disappear only 5 seconds before impact, at an altitude of only
about 200 km above the 1-bar pressure level: it may well start its bolide
phase while still in view. Furthermore, any sufficiently dense post-impact
plume will have to rise only a few hundred kilometers to be visible from Earth.

Simulations for 2-4 km fragments by Mark Boslough (Sandia National
Laboratories) indicate when the fireball resulting from an impact cools it
would form a debris cloud that will rise hundreds of kilometers above the
Jovian cloudtops, and would enter sunlight within minutes of the impact. The
arrival time of this giant cloud into sunlight would provide data on its
trajectory, which in turn would help us know how big the comet fragment was.
It is possible that it would be big (bright) enough to be seen by amateurs
[42,43].

If the fragments are 2-4 km in diameter then the probability is very high
that these effects will be visible for some of the later impacts (e.g. W and R,
visible from Hawaii, S, visible from India and the far East, Q1 and Q2, visible
from Africa, parts of eastern Europe and the Middle East, L, Brazil and West
Africa, K, South Pacific and Australia, and maybe even V, on the final night,
visible in the Western half of the U.S.). Observers in these locations are
encouraged to anticipate the possibility of seeing the fireball within tens of
seconds after the impact, and a few minutes later after it has cooled,
condensed, and entered the sunlight. The apparent visible magnitude of any
fireball will be similar to that of a Galilean satellite at best, but it would
appear redder. They would be much brighter at infrared wavelengths.

The following predictions by Mordecai-Mark Mac Low (University of Chicago)
are based on simulations for 1 km fragments: Each comet fragment will enter
the Jupiter's atmosphere at a speed of 130,000 mph (60 km/s). At an altitude
of 100 km above the visible cloud decks, aerodynamic forces will overwhelm the
material strength of the fragment and tear it apart. Five seconds after
entry, the comet fragment would deposit its kinetic energy of around 10^28 ergs
(equivalent to around 200,000 megatons of TNT) at 100-150 km below the cloud
layer [19]. Bigger fragments will have more energy and go deeper.

The hot (30,000 K) gas resulting from a 1 km stopped comet will explode,
forming a fireball similar to a nuclear explosion, but much larger. The
visible fireball may only rise 100 km or so above the cloudtops in this case.
Above that height the density may drop so that it will become transparent.
The fireball material will continue to rise, reaching a height of perhaps
1000 km before falling back down to 300 km. The fireball will spread out over
the top of the stratosphere to a radius of 2000-3000 km from the point of
impact. The top of the resulting shock wave will accelerate up out of the
Jovian atmosphere in less than two minutes, while the fireball will be as
bright as the entire sunlit surface of Jupiter for around 45 sec [18]. The
fireball will be somewhat red, with a characteristic temperature of 2000 K -
4000 K (redder than the sun, which is 5800 K). Virtually all of the shocked
cometary material will rise behind the shock wave, leaving the Jovian
atmosphere and then splashing back down on top of the stratosphere at an
altitude of 300 km above the clouds [unpublished simulations by Mac Low &
Zahnle]. Not much mass is involved in this splash, so it will not be directly
observable. The splash will be heavily enriched with cometary volatiles such
as water or ammonia, and so may contribute to significant high hazes.

Meanwhile, the downward moving shock wave will heat the local clouds,
causing them to buoyantly rise up into the stratosphere. This will allow
spectroscopists to attempt to directly study cloud material, a unique
opportunity to confirm theories of the composition of the Jovian clouds.
Furthermore, the downward moving shock may drive seismic waves (similar to
those from terrestrial earthquakes) that might be detected over much of the
planet by infrared telescopes in the first hour or two after each impact.
The strength of these two effects remains a topic of research. The
disturbance of the atmosphere will drive internal gravity waves ("ripples in
a pond") outwards. Over the days following the impact, these waves will
travel over much of the planet, yielding information on the structure of
the atmosphere if they can be observed (as yet an open question).

The "wings" of the comet will interact with the planet before and after
the collision of the major fragments. The so-called "wings" are defined to
be the distinct boundary along the lines extending in both directions from
the line of the major fragments; some call these 'trails'. Sekanina, Chodas
and Yeomans have shown that the trails consist of larger debris, not dust:
5-cm rock-sized material and bigger (boulder-sized and building-sized).
Dust gets swept back above (north) of the trail-fragment line due to solar
radiation pressure. The tails emanating from the major fragments consist of
dust being swept in this manner. Only the small portion of the eastern
debris trail nearest the main fragments will actually impact Jupiter,
according to the model, with impacts starting only a week before the major
impacts (July 8, 1994). The western debris trail, on the other hand, will
impact Jupiter over a period of months following the main impacts, with the
latter portion of the trail actually impacting on the front side of Jupiter
as viewed from Earth [20].

The injection of dust from the wings and tail into the Jovian system
may have several consequences. First, the dust will absorb many of the
energetic particles that currently produce radio emissions in the Jovian
magnetosphere. The expected decline and recovery of the radio emission may
occur over as long as several years, and yield information on the nature and
origin of the energetic particles. Second, the dust may actually form a
second faint ring around the planet.

At the University of Oregon astronomers plan to monitor the linearly and
circular polarized light from Jupiter. They have a sensitive detector that
can detect changes of one part in 10^5 and they expect to see some kind of
change in the polarization signal as the magnetic field of Jupiter is
disrupted by the impacts.

Due to the great distance between Jupiter and the Earth, the comet
poses no threat to Earth. However, Eugene Shoemaker says that if a similar
comet crashed on Earth it would be catastrophic: "...we're talking about a
million megatons of kinetic energy. We're talking about the kind of event
that is associated with mass extinction of species on Earth; really and truly
a global catastrophe. It might not take out the human race but it would
certainly be very bad times." (CNN, Headline News)

One might be able to detect atmospheric changes on Jupiter using
photography or CCD imaging. It is important, however, to observe Jupiter
for several months in advance in order to know which features are due to
impacts and which are naturally occurring. It appears more and more likely
that most effects will be quite subtle. Without a large ( > 15" ?) telescope
and good detector, little is likely to be seen.

It is possible that the impacts may create a new, temporary storm at the
latitude of the impacts. Modeling by Harrington et al. suggests this is
possible [30]. The fragments of comet Shoemaker-Levy 9 will strike just
south of the South South Temperate Belt of Jupiter. If the nuclei penetrate
deep enough, water vapor may shoot high into the atmosphere where it could
turn into a bluish shroud over a portion of the South South Temperate Zone
[31].

Impacts of the largest fragments may create one or two features. A spot
might develop that could be a white or dark blue nodule and would likely have
a maximum diameter of 2,000 km to 2,500 km which in a telescope would be 1
to 1.5 arcseconds across. This feature would be very short-lived with the
impact site probably returning to normal after just a few rotations of
Jupiter. A plume might also develop that would look dark against the South
Temperate Zone's white clouds or could appear as a bright jet projected from
Jupiter limb [31]. The table below shows the approximate sizes of features
that already exist on Jupiter for comparison.

Below is a list of files available at ftp.tamu.edu in the /pub/comet
directory that may be helpful in identifying features on Jupiter:

tracker3.zip MSDOS program that displays the location of impact sites
and features of Jupiter
jupe.description Description of a PC program showing features of Jupiter
jul1994.transit Transit Times for Red Spot and White Spots for July 1994
jul1994.moons Jovian Moon events for July 1994 (Shadows, Eclipses, etc.)

Also, there are little anticyclonic ovals at jovicentric latitudes of
about -45 degrees which are typical of the South South Temperate domain
and are about 3500 km in diameter. There are usually 6 or 7 around the planet
and they move with the South South Temperate current, i.e. faster than BC and
DE. See Sky & Telescope for a CCD image of these ovals by Don Parker [38].

NASA's coverage of the impact of Comet P/Shoemaker-Levy 9 during
the week of July 16-22 includes a series of live, televised press briefings
and a 24-hour newsroom operation at the Goddard Space Flight Center (GSFC),
Greenbelt, Md. The briefing panels will include Comet co-discoverers Drs.
Eugene and Carolyn Shoemaker and David Levy on most days as well as scientists
presenting images and information from the Hubble Space Telescope and other
spacecraft. Dr. Lucy McFadden will have a round-up of observations from
ground-based observatories around the world. The program and briefing
schedule follows:

Note: The above times are dependent on the STS-65 mission schedule. If
there is a change in the launch or landing time of the Shuttle, the program
times will change. Video Uplink Schedule: NASA will provide feeds of b-roll
and animation of the comet impacts with Jupiter on the following schedule:
July 15, 1:00 p.m. EDT. NASA TV is carried on Spacenet 2, transponder 5,
channel 9, 69 degrees West, transponder frequency is 3880 MHz, audio
subcarrier is 6.8 MHz, polarization is horizontal.

CNN also plans to offer live coverage at 10:00pm EDT on July 16th with
comments and explanation from scientists at Goddard and the Space Telescope
Science Institute. CNN plans live coverage of at least the first 2 briefings
(Sunday and Monday) and then TBA depending on what the images look like. CNN
also plans to have live reports at 9am and noon EDT throughout the week with a
daily comet report for release in the afternoon.

Note also that your local PBS station will normally pick up one
of the live feeds listed below so that you will see it at 10:30-11:30
local time. If you have a TVRO satellite dish you can watch it 5 times.

Also, there is a gathering of professional and amateur astronomers every
week on the IRC (Internet Relay chat) channel #Astronomy for real time
discussions. Friday and Sunday sessions are held at 20:00 UT. There may be
continuous discussion/updates on the IRC channel #Astronomy during the impacts.

The cutoff of radio emissions due to the entry of cometary dust into the
Jovian magnetosphere during the weeks around impact may be clear enough to be
detected by small radio telescopes. Furthermore, impacts may be directly
detectable in radio frequencies. Some suggest to listen in on 15-30 MHz
during the comet impact. So it appears that one could use the same antenna
for both the Jupiter/Io phenomenon and the Jupiter/comet impact. There is
an article in Sky & Telescope magazine which explains how to build a simple
antenna for observing the Jupiter/Io interaction [4,24,25].

For those interested in radio observations during the SL9 impact,
Leonard Garcia of the University of Florida has made some information
available. The following files are available via anonymous ftp on the
University of Florida, Department of Astronomy site astro.ufl.edu in the
/pub/jupiter directory:

README.DOC Explanation of predicted Jupiter radio storms tables
jupradio.txt Jovian Decametric Emission and the SL9/Jupiter Collision
july94.txt Tables of predicted Jupiter radio storms for July 1994

The antenna required to observe Jupiter may be as simple as a dipole
antenna constructed with two pieces of wire 11 feet 8.4 inches in length,
connected to a 50 ohm coax cable. This antenna should be laid out on a
East-West line and raised above the ground by at least seven feet. A
Directional Discontinuity Ring Radiator (DDRR) antenna is also easy to
construct and can be made from 1/2 inch copper tubing 125.5 inches in
length (21Mhz). The copper tube should be bent into a loop and placed 5
inches above a metallic screen. A good preamp is required for
less sensitive shortwave receivers [39].

Society of Amateur and Radio Astronomers (SARA) say that amateur radio
astronomers may have to wait approximately three hours after impact for the
impact sites to rotate to the central meridian of Jupiter before anything
unusual is detected. This wait is typical due to the Jovian decametric
synchrotron emissions being emitted as a beam of radiation. Due to the
large time differential from impact to radio observations any disturbance may
have settled and not be detected. SARA suggest that the radio observer
begin the watch approximately 30 minutes before the fragments hit to four
hour after.

One may be able to witness the collisions indirectly by monitoring the
brightness of the Galilean moons that may be behind Jupiter as seen from
Earth. One could monitor the moons using a photometer, a CCD camera. However,
current calculations suggest that the brightenings may be as little as 0.05%
of the sunlit brightness of the moon [18]. If a moon can be caught in eclipse
but visible from the earth during an impact, prospects will improve
significantly. According to current predictions, the only impact certain to
occur during a satellite eclipse is K=12 with Europa eclipsed. However, H=14
and W=1 impact only about 2 sigma after Io emerges from eclipse at longitude
20 deg, and B=20, E=17 and F=16 impact 0.5-2 sigma after Amalthea emerges from
eclipse at longitude 34 deg. See also Q2.1 of this FAQ for satellite locations.

The following files contain information concerning the reflection of
light by Jupiter's moons and are available at SEDS.LPL.Arizona.EDU :

galsat53.zip MSDOS Program that Displays relative positions of
Jupiter's Moons during times of impact
impact_24apr.ps PostScript Plot of impact times at satellite availability

Also, monitoring the eclipses of the Galilean satellites after the
impacts may yield valuable scientific data with the moons serving as
sensitive probes of any cometary dust in Jupiter's atmosphere. The geometry
of the eclipses is such that the satellites pass through the shadow at
roughly the same latitude as the predicted comet impacts. There is an
article in the first issue of CCD Astronomy involving these observations.
The article says that if the dust were to obscure sunlight approximately
120 kilometers above Jupiter's cloud tops, Io could be more that 3 percent
(0.03 magnitudes) fainter than normal at mideclipse [40].

Comet Shoemaker-Levy 9 is actually in a temporary orbit of Jupiter,
which is most unusual: comets usually just orbit the Sun. Only two comets
have ever been known to orbit a planet (Jupiter in both cases), and this was
inferred in both cases by extrapolating their motion backwards to a time
before they were discovered. S-L 9 is the first comet observed while orbiting
a planet. Shoemaker-Levy 9's previous closest approach to Jupiter (when it
broke up) was on July 7, 1992; the distance from the center of Jupiter was
about 96,000 km, or about 1.3 Jupiter radii. The comet is thought to have
reached apojove (farthest from Jupiter) on July 14, 1993 at a distance of
about 0.33 Astronomical Units from Jupiter's center. The orbit is very
elliptical, with an eccentricity of over 0.998. Computations by Paul Chodas,
Zdenek Sekanina, and Don Yeomans, suggest that the comet has been orbiting
Jupiter for 20 years or more, but these backward extrapolations of motion are
highly uncertain. See "elements.*" and "ephemeris.*" at SEDS.LPL.Arizona.EDU
in /pub/astro/SL9/info for more information.

In the abstract "The Orbit of Comet Shoemaker-Levy 9 about Jupiter"
by D.K. Yeomans and P.W. Chodas (1994, BAAS, 26, 1022), the elements
for the brightest fragment Q are listed. These elements are Jovicentric
and for Epoch 1994Jul15 (J2000 ecliptic):

The comet broke apart due to tidal forces on its closest approach to
Jupiter (perijove) on July 7, 1992 when it passed within the theoretical
Roche limit of Jupiter. Shoemaker-Levy 9 is not the first comet observed
to break apart. Comet West shattered in 1976 near the Sun [3]. Astronomers
believe that in 1886 Comet Brooks 2 was ripped apart by tidal forces near
Jupiter [2]. Several other comets have also been observed to have split
[41].

Furthermore, images of Callisto and Ganymede show crater chains which
may have resulted from the impact of a shattered comet similar to Shoemaker-
Levy 9 [3,17]. The satellite with the best example of aligned craters is
Callisto with 13 crater chains. There are three crater chains on Ganymede.
These were first thought to be from basin ejecta; in other words secondary
craters [27]. See SEDS.LPL.Arizona.edu in /pub/astro/SL9/images for images
of crater chains (gipul.gif and chain.gif).

There are also a few examples of crater chains on our Moon. Jay Melosh
and Ewen Whitaker have identified 2 possible crater chains on the moon which
would be generated by near-Earth tidal breakup. One is called the "Davy
chain" and it is very tiny but shows up as a small chain of craters aligned
back toward Ptolemaeus. In near opposition images, it appears as a high
albedo line; in high phase angle images, you can see the craters themselves.
The second is between Almanon and Tacitus and is larger (comparable to the
Ganymede and Callisto chains in size and length). There is an Apollo 11
image of a crater chain on the far side of the moon at SEDS.LPL.Arizona.edu
in /pub/astro/SL9/images (moonchain.gif).

Using measurements of the length of the train of fragments and a model
for the tidal disruption, J.V. Scotti and H.J. Melosh have estimated that the
parent nucleus of the comet (before breakup) was only about 2 km across [13].
This would imply that the individual fragments are no larger than about 500
meters across. Images of the comet taken with the Hubble Space Telescope in
July 1993 indicate that the fragments are 3-4 km in diameter (3-4 km is an
upper limit based on their brightness; the fragments have visual magnitudes of
around 23). A more elaborate tidal disruption model by Sekanina, Chodas and
Yeomans [20] predicts that the original comet nucleus was at least 10 km in
diameter. This means the largest fragments could be 3-4 km across, a size
consistent with estimates derived from the Hubble Space Telescope's July 1993
observations.

The new images, taken with the Hubble telescope's new Wide Field and
Planetary Camera-II instrument in 1994, have given us an even clearer view
of this fascinating object, which should allow a refinement of the size
estimates. Some astronomers now suggest that the fragments are about 1 km or
smaller. In addition, the new images show strong evidence for continuing
fragmentation of some of the remaining nuclei, which will be monitored by
the Hubble telescope over the next two weeks. One can get an idea of the
relative sizes of the fragments by considering the relative brightnesses:

The angular length of the train was about 51 arcseconds in March 1993
[2]. The length of the train then was about one half the Earth-Moon
distance. In the day just prior to impact, the fragment train will stretch
across 20 arcminutes of the sky, more that half the Moon's angular diameter.
This translates to a physical length of about 5 million kilometers. The
train expands in length due to differential orbital motion between the first
and last fragments. Below is a table with data on train length based on
Sekanina, Chodas, and Yeomans's tidal disruption model:

The Hubble Space Telescope, like earthlings, will not be able to see the
collisions but will be able to monitor atmospheric changes on Jupiter. The
impact points are favorable for viewing from spacecraft: it can now be stated
with certainty that the impacts will all be visible to Galileo, and now at
least some impacts will be visible to Ulysses. Although Ulysses does not have
a camera, it will monitor the impacts at radio wavelengths.

Galileo will get a direct view of the impacts rather than the grazing
limb view previously expected. The Ida image data playback was scheduled to
end at the end of June, so there should be no tape recorder conflicts with
observing the comet fragments colliding with Jupiter. The problem is how to
get the most data played back when Galileo will only be transmitting at 10
bps. One solution is to have both Ulysses and Galileo record the event and
and store the data on their respective tape recorders. Ulysses observations
of radio emissions data will be played back first and will at least give
the time of each comet fragment impact. Using this information, data can
be selectively played back from Galileo's tape recorder. From Galileo's
perspective, Jupiter will be 60 pixels wide and the impacts will only show
up at about 1 pixel, but valuable science data can still collected in the
visible and IR spectrum along with radio wave emissions from the impacts.

The impact points are also viewable by both Voyager spacecraft,
especially Voyager 2. Jupiter will appear as 2.5 pixels from Voyager 2's
viewpoint and 2.0 pixels for Voyager 1. However, it is doubtful that the
Voyagers will image the impacts because the onboard software that controls
the cameras has been deleted, and there is insufficient time to restore and
test the camera software. The only Voyager instruments likely to observe
the impacts are the ultraviolet spectrometer and planetary radio astronomy
instrument. Voyager 1 will be 52 AU from Jupiter and will have a near-limb
observation viewpoint. Voyager 2 will be in a better position to view the
collision from a perspective of looking down on the impacts, and it is also
closer at 41 AU.

Observation forms by Steve Lucas are available via ftp at
oak.oakland.edu in the /pub/msdos/astrnomy directory. These forms also
contain addresses of "Jupiter Watch Program" section leaders. jupcom02.zip
contains Microsoft Write files. The Association of Lunar and Planetary
Observers (ALPO) will also distribute a handbook to interested observers.
The handbook "The Great Crash of 1994" is available for $10 by ALPO Jupiter
Recorder, Phillip W. Budine, R.D. 3, Box 145C, Walton, NY 13856 U.S.A. The
cost includes printing, postage and handling.

John Rogers, the Jupiter Section Director for the British Astronomical
Association, will be collecting data from regular amateur Jupiter observers
in Britain and worldwide. He can be reached via email (jr@mole.bio.cam.ac.U)
or fax (UK [223] 333840). The Society of Amateur and Radio Astronomers (SARA)
is collecting radio observations of the events. Observations can be sent to
the chairman of the SARA Comet Watch Committee: Tom Crowley, 3912 Whittington
Drive, Altanta, GA 30342, EMAIL : 70651.2032@compuserve.com.

The SL9 educator's book put out by JPL is in the /pub/astro/SL9/EDUCATOR
directory of SEDS.LPL.Arizona.edu. There are two technical papers [18,19]
on the atmospheric consequences of the explosions available at
oddjob.uchicago.edu in the /pub/jupiter directory. There are some PostScript
images and text files involving the results of fireball simulations by
Sandia National Laboratories at ftp.tamu.edu (128.194.103.11) in the
/pub/comet/sandia directory.

SEDS (Students for the Exploration and Development of Space) has set up
an anonymous account which allows you to use "lynx" - a VT100 WWW browser.
To access this service, telnet to SEDS.LPL.Arizona.EDU and login as "www"
(no password required). This will place you at the SEDS home page, from
which you can select Shoemaker-Levy 9. A similar "gopher" interface is
available at the same site. Just login as "gopher".